3.1 Preparation and properties analysis of MSC
Figure 1(A) presented the preparation process of MSC. SA was used as a type of shell material to prepare microcapsules. FA and urea were used as water-soluble core materials. A mixture of FA, urea and liquid paraffin was prepared by stirring to form a homogeneous emulsion, and then homogenated with SA solution to form the structure of W/O/W. During this process, the characteristic of surfactant and the use of stirring rate were important to the preparation of the emulsion. In addition, the MSC can be prepared by the method of sharp-hole coagulation bath due to the massive amount requirement. The emulsifying emulsion was made into liquid drops through a sharp-hole device and dropped into the coagulation bath. The wall material of the liquid drop reacts with the coagulation bath to form water-insoluble microcapsules. The MSC was finally formed by CaCl2 through ionic crosslinking mechanism which was presented in Fig. 1(B) [23–26]. The particle size of the MSC was ranged between 1.58 and 2.14 mm (Fig. 1(C)), which was consistent with the urea particle size used in actual agricultural production. In addition, it can be seen that the microcapsules were spherical in size.
The test results of electrical conductivity for soil treated with different MSC were shown in Fig. 2(A). It was shown that the conductivity of all experimental groups was higher than that of the control group after treatment. This indicated that the concentration of soluble ions in the dry sand was increased with the addition of different MSC. Thus, the conductivity of soil was improved, and the ion exchange capacity and the element diversity of soil can be effectively improved. Ca2+ was used in the preparation of microcapsules, and this may be the reason for the increase of ion concentration in soil. Ion exchange ability is an important parameter for soil, which can be used to regulate soil environment and promote crop growth. When Ca2+ was applied into soil, it was released into the soil to regulate the type and concentration of ions. The ability of controlling leaching loss of microencapsulated materials reflects the ability of preservation the nutrition for MSC. The relevant experimental data were given in Fig. 2(B). Compared with the control group, four groups of experimental groups can effectively improve the effect of water resistance. This may be ascribed to the following two reasons: (1) the natural polymer of microcapsule shell possesses certain water absorption ability, and thus it may retain some nutrients due to its water absorption features; (2) a relatively dense spatial structure may be existed in the microcapsules, and it can prevent leaching phenomenon. The sustained release curves of MSC with different core materials were presented in Fig. 2(C). It can be seen that the prepared microcapsules owned good sustained-release behavior. Its sustained-release time was at least 600 h (about 25 d), which can cover the growth range of most crops. The biodegradation curves of MSC were illustrated in Fig. 2(D). A large mass loss occurred in the first 50 h, which may be the degradation process of outer shell of sodium alginate. The first mass reduction was resulted from the degradation of its outer layer. With the decrease of the outer shell, the protective structure of the core was destroyed. This may lead to the loss of the core materials along with the degradation of the outer shell.
Figure 3 showed the breeding and planting process of cabbages. As shown in Fig. 3(a), after the buds grew in the plug, the seedlings which were far away from each other and owned the same growth vigor were selected as the seedling raising objects. Other buds were cut off for transplanting preparation. In Fig. 3(b), after about two weeks, the seedlings with similar growth vigor were selected and transplanted into the breeding pot, and the water was poured through the breeding basin every afternoon to keep the soil’s moisture. The insects were caught and the physiological status was checked every three days to ensure the normal growth of cabbages. Figure 3(c) and (d) presented the growth status of these cabbages after three and four weeks. Compared with the control group, CK(Fig. 3(A)), the growth status of the four experimental groups, for examples W1H5,W2H5,W3H5,W4H5(Fig. 3(B)-(E)), was significantly improved, which proved that the microencapsulated soil conditioner could improve the soil structure and had the effect of promoting plant growth. During the above four periods, the nitrogen balance index, physical and chemical properties of soil and physiological growth performance were monitored, and the photosynthesis intensity and root growth were measured and analyzed.
3.2 Physiological growth curves
High throughput plant phenotype technology may be used to continuously track and measure physiological parameters such as plant height and leaf width without physical damage [27]. It owned the advantages of accuracy, objectivity and efficiency, and can provide strong support for related research. However, the current reports on the research of plant phenotype equipment in the field of physiological growth performance were not common. In this experiment, the plant phenotypic equipment was used to continuously track and measure the growth process of cabbages, as shown in Fig. 4(A) and (B), which was the photos taken from 0° and 90° direction, respectively. It can be seen from Fig. 4 that the plant height and leaf width of cabbages showed a significant increasing trend, and it may provide relatively objective basis for the relevant data of crop growth performance [28].
Figure 5 presented the data of leaf width and plant height of the control and experimental groups which may express the growth trend of cabbages [29]. It can be seen from Fig. 5(a) that there was no significant difference for leaf width in the first 7 d. This might be due to the fact that the embryo of the seed itself provided enough nutrients for the growth of cabbages, and the plant height was about 6–8 cm (Fig. 5(b)) [30]. Compared with the control group, the growth trend of sustained-release fertilizer and traditional fertilizer groups was better. After 14 d, it was found that the plant height (about 14 cm) of traditional fertilization group was better than that of sustained-release fertilizer group (about 13 cm) and the control group (about 12 cm). However, the leaf width (about 25–27 cm) of the sustained-release fertilizer group was better than that of the traditional fertilizer group (about 25 cm) and the control group (about 23 cm). Under the condition of insufficient seed nutrition, the content of nitrogen fertilizer can directly affect the physiological growth performance of cabbages. However, in 15–25 d, it was found that the cabbages in the sustained-release fertilizer group continued to grow rapidly, and the leaf width increased from about 27–30 to 37–44 cm. In contrast, the traditional fertilizer group grew from 25 to 31 cm, and the control group grew from 24 to 31 cm.
These results showed that sustained-release fertilizer could continuously provide fertility for cabbage growth compared to traditional fertilization. FA could not only promote the growth of roots, but also protect the decomposition of soil microorganisms in the nitrogen fertilizer position, thus prolonging the existence time of nitrogen fertilizer in the soil. Calcium ions may activate the cell wall of plant root and enhance its ability of absorbing nutrients and water. Therefore, the final growth trend was that W3Hn was greater than W4Hn, and W4Hn were greater than that of W2Hn, traditional group and the control group [31]. Results showed that with the same urea concentration, the effect of sustained-release microcapsules was conducive to the growth of cabbages, while FA and calcium ion concentration were conducive to improve the utilization rate of urea. Compared with Fig. 5(a,c) and (b,d), it could be seen that the growth of cabbages buried in 2 cm was better than those buried in 5 cm. This might be related to the shallower root system and better lateral root, which could not extend down to absorb more nutrients. Therefore, sustained-release fertilizer and shallow layer were beneficial to the growth of cabbages.
3.3 Nitrogen content of plant leaves and photosynthesis
Nitrogen balance index (NBI) was the ratio of chlorophyll (CHL) to flavonoid (flav), which was the main index reflecting the growth of cabbages [32–34]. Flavonoids could resist oxidation and scavenge active hydrogen, while chlorophyll was the main substance for photosynthesis. At the beginning, NBI was similar because the growth of cabbages using the nutrients in the embryo was not related to the external fertilization and NBI was related to the germination cycle of plants (Fig. 6). On the 4th day, NBI value of all cabbages reached the maximum value, and this was ascribed to less leaves and more nutrients in this plant. From the 4th to 7th days, the growth of cabbages was promoted and the chlorophyll content in the leaves was increased because of the external nitrogen supply. The NBI of the sustained-release fertilizer group was higher than that of the traditional fertilization and the control groups, which illustrated that the sustained-release fertilizer groups showed better effect of providing nitrogen fertilizer than that of the traditional fertilization groups. During the growth process from the 7th to 20th days, the NBI of the sustained-release fertilizer group was much higher than that of the traditional fertilizer and the control groups. The NBI of the traditional fertilizer and the control groups showed a continuous downward trend, and on the contrary, the sustained-release fertilizer group remained high and there was an upward fluctuation. The traditional fertilization could only provide nutrients in a short period of time and owned no ability of sustainable fertilization, while sustained-release fertilizer could continuously provide the nitrogen fertilizer needed during the growth process of cabbages.
Net photosynthetic rate could be used as a direct indicator of CO2 to sugar conversion. It can directly reflect the indicators of cabbage production and growth capacity, which were related to the chlorophyll content in cabbage leaves (Fig. 7(a)). Results showed that the net photosynthetic rate of sustained-release fertilizer group was much higher than that of traditional fertilizer group. In addition, the nitrogen content could directly affect the chlorophyll content in the leaves [35–38]. The sustained-release fertilizer had the ability to continuously supply nitrogen, and thus it may maintain sufficient chlorophyll content in the leaves of cabbages and the growth of cabbages was maintained.
Intercellular carbon dioxide concentration in the morning or noon could be used as indirect evidence to reflect crop photosynthesis (Fig. 7(b)) [39]. Under the condition of the same light and carbon dioxide concentration, the higher the intercellular carbon dioxide concentration was, the lower the carbon dioxide consumption for photosynthesis was. Therefore, there was a negative correlation between the intercellular carbon dioxide concentration and the photosynthesis rate [40].
The transpiration rate of crops was the driving force of crop water absorption (Fig. 7(c)) [41]. Water was also the main raw material for photosynthesis of cabbage to prepare nutrients, so the transpiration of crops was also an important index for the growth process of cabbages [42]. The transpiration rate of the sustained-release fertilizer group was higher than that of the traditional fertilizer group, which was consistent with the above trend of photosynthesis rate. It proved that the sustained-release fertilizer group owned better physiological growth performance compared to the traditional fertilizer and the control groups.
3.4 Root analysis and yield of cabbages
As the main part of absorbing soil nutrients and water, root analysis may reflect the growth trend [43–45]. By comparing the total length, total surface area, average root diameter and dry weight of roots in Fig. 8, the roots of the experimental group treated with sustained-release fertilizer were more developed, which proved that it had better physiological performance.
From the order of total root length: W3H2 (182.7 cm) > W3H5 (162.5 cm) > W4H2 (142.2 cm) > W4H5 (130.5 cm) > W2H5 (118.2 cm) > W2H2 (115.7 cm) > W1H5 (87.0 cm) > W1H2 (82.7 cm) > CK (55.6 cm), it can be seen that the traditional fertilization had a promoting effect on root growth, and the length of root was increased by 31.4 cm (W1H5) and 27.1 cm (W1H2), respectively. In addition, compared with CK, the application of sustained-release fertilizer can increase the total length from 60.1 cm (W2H2) to 127.1 cm (W3H2), which had a more obvious effect on the root. In the same way, the trend of total surface area was similar to that of root length, indicating that sustained-release fertilizer may effectively improve the physiological growth performance of the cabbages. This illustrated that this novel soil conditioner presented a good prospect in practical application.
Figure 9 presented the wet (a) and dry (b) weight of mature cabbages after picking and drying. It can be seen that the yield of cabbage treated with nitrogen fertilizer was significantly higher than that of the control group, indicating that nitrogen fertilizer played an extremely important role in the yield of cabbage. Compared to the traditional fertilization group, the sustained-release fertilizer group exhibited higher yield. For example, when the fertilizer was applied at 5 cm, the order of yield for each group was W3H5 (8.01 g) > W4H5 (6.90 g) > W2H5 (5.18 g) > W1H5 (5.07 g) > CK (4.01 g). This illustrated that the sustained-release nitrogen fertilizer may provide more sufficient fertilizer compared to the traditional fertilization method.
The total nitrogen content and organic content of the soil after planting cabbages were detected and analyzed (Fig. 9(c)). It can be found that compared with the nitrogen content of 0.65g/kg in the control group, the soil of other groups using fertilizer contained significantly more nitrogen content. The order of them was W3H2(0.786g/kg) > W2H2(0.781g/kg) > W1H2(0.735g/kg) > W4H2(0.719g/kg), indicating that the use of nitrogen fertilizer can greatly increase the nitrogen content in soil, which can improve crop yield and related physiological growth performance. Similarly, through the analysis of organic matter, the order of its organic matter content was W3H2 > W2H2 > W1H2 > W4H2 > CK, which was mainly because the microencapsulated soil conditioner capsule can be degraded as organic matter and increase the organic matter content in the soil, so as to improve crop yield.